The problems of game state synchronisation; also has a nice bit about simulation vs. visualisation timescales. The unstated lesson here is that you want a more graceful way of recovering from desyncs than just completely stopping the game!

"This paper explains the design architecture, implementation, and some of the lessons learned creating the multiplayer (networking) code for the Age of Empires 1 & 2 games; and discusses the current and future networking approaches used by Ensemble Studios in its game engines." Circa 2001.